The seasonality of postoperative infection in spine surgery

Studies from many disciplines have found an association with the summer months, elevated temperature, humidity, and an increased rate of infection. The "July effect," a hypothesis that the inexperience of new house staff at the beginning of an academic year leads to an increase in wound complications, has also been considered. Finally, an increase in trauma-related admissions in the summer months is likely to result in an increased incidence of postoperative infections. Two previous studies revealed mixed results concerning perioperative spinal wound infections in the summer months. The purpose of this study was to determine the months and/or seasons of the year that display significant fluctuation of postoperative infection rate in spine surgery. Based on the idea that infection rates are susceptible to seasonal factors, the authors hypothesized that spinal infections would increase during the summer months.

Inclusion criteria were all spine surgery cases at a single tertiary referral institution between January 2005 and December 2009; 8122 cases were included. Patients presenting with a contaminated wound or active infection were excluded. Infection rates were calculated on a monthly and seasonal basis and compared.

A statistically significant increase in the infection rate was present on both a seasonal and monthly basis (p = 0.03 and p = 0.024) when looking at the seasonal change from spring to summer. A significant decrease in the infection rate was seen on a seasonal basis during the change from fall to winter (p = 0.04). The seasonal rate of infection was highest in the summer (4.1%) and decreased to the lowest point in the spring (2.8%) (p = 0.03).

At the authors' institution, spine surgeries performed during the summer and fall months were associated with a significantly higher incidence of wound infection compared with the winter and spring. These data support the existence of a seasonal effect on perioperative spinal infection rates, which may be explained by seasonal variation in weather patterns and house staff experience, among other factors.